1. Field of the Invention
This invention relates to compression ignition engines and in particular to the operation of high-speed compression ignition engines in such manner as to reduce the amounts of oxides of nitrogen in the exhaust gases.
2. Prior Art
As result of increasingly stringent federal standards with respect to emmissions from automobile exhausts, alternative power plants for automobiles are being investigated. One popular alternative power plant is the compression ignition engine, commonly known as the Diesel engine.
The Diesel engine has several advantages over conventional spark ignition engines. In particular, Diesel engines burn heavier fuel which is cheaper than gasoline, they have a higher thermal efficiency than spark ignition engines, and they have significantly lower emmissions in some respects than comparable spark ignition engines. While carbon monoxide emmissions are low because the Diesel engine operates with excess air, and hydrocarbons are normally a small constituent of Diesel exhaust, Diesel engines characteristically produce unacceptably high amounts of oxides of nitrogen (NO.sub.x) and therefore are presently unable to meet government standards with respect to NO.sub.x emmissions for automobiles.
The standard Diesel engine used in some automobiles and most trucks today is a four-stroke or four cycle engine. In the first or intake stroke, the intake valve opens and the piston decends to draw fresh air into the cylinder. In the second or compression stroke, the intake valve closes and the piston rises to compress the air which becomes heated. At the end of the compression stroke, fuel is injected into the cylinder and burns.
In the third or expansion stroke, the burning mixture expands and forces the piston down. At this time both the intake and the exhaust valves are closed.
In the fourth or exhaust stroke, the exhaust valve opens and the burned gases are forced out of the cylinder by the rising piston.
Since the working fluid, namely air, is a compressible gas that enters and leaves the cylinder in more than an instantaneous period of time, the closing of the exhaust valve at the end of the exhaust stroke typically occurs subsequent to the opening of the intake valve at the beginning of the air intake stroke. In other words, the exhaust valve remains open until after the piston reaches top dead center, and the intake valve opens before the piston reaches top center. The reason for this "valve overlap" is to effect a more thorough scavenging of the exhaust gases from the cylinder, which brings about an increase in power out of proportion to the amount of air involved.
When the exhaust stroke begins and the exhaust valve opens, the motion of the exhaust gases is started by the cylinder pressure existing when the exhaust valve is opened and is promoted by the piston motion during the exhaust stroke. The scavenging of exhaust gases tends to continue during and after the top center period. Therefore, the intake valve is opened to allow fresh air to enter the cylinder to displace the last traces of exhaust gases in the cylinder and a necessary result of this procedure is that a certain amount of fresh air is drawn through the cylinder and out past the exhaust valve where it mixes with the exhaust gases.
It is believed that the occurrence of this valve overlap, during which fresh air is drawn in through the intake valve and out through the exhaust valve, is a major cause of the formation of unacceptable amounts of NO.sub.x in the exhaust gas of a Diesel engine.